CVE-2019-20175 in QEMU
Summary
by MITRE
** DISPUTED ** An issue was discovered in ide_dma_cb() in hw/ide/core.c in QEMU 2.4.0 through 4.2.0. The guest system can crash the QEMU process in the host system via a special SCSI_IOCTL_SEND_COMMAND. It hits an assertion that implies that the size of successful DMA transfers there must be a multiple of 512 (the size of a sector). NOTE: a member of the QEMU security team disputes the significance of this issue because a "privileged guest user has many ways to cause similar DoS effect, without triggering this assert."
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Analysis
by VulDB Data Team • 06/27/2024
The vulnerability identified as CVE-2019-20175 resides within the QEMU virtualization platform's IDE subsystem, specifically in the ide_dma_cb() function located in hw/ide/core.c. This issue affects QEMU versions ranging from 2.4.0 through 4.2.0, representing a significant portion of the virtualization software's history. The flaw manifests when a guest operating system attempts to execute a special SCSI_IOCTL_SEND_COMMAND ioctl call, which triggers an assertion failure in the host system's QEMU process. The assertion specifically requires that successful DMA transfer sizes must be multiples of 512 bytes, corresponding to the standard sector size used in storage systems. This constraint creates a potential denial of service condition where a malicious guest can cause the host QEMU process to crash and terminate unexpectedly.
The technical exploitation of this vulnerability involves leveraging the SCSI_IOCTL_SEND_COMMAND ioctl interface to manipulate DMA transfer operations in a way that violates the imposed sector size requirement. When the assertion fails, it indicates that the DMA transfer size does not conform to the expected 512-byte multiple, causing the QEMU process to abruptly terminate. This represents a classic denial of service scenario where an attacker can disrupt the virtualization environment without necessarily requiring root privileges or complex exploitation techniques. The vulnerability demonstrates a lack of proper input validation within the DMA handling code path, where the system assumes all valid DMA transfers will conform to specific size constraints without adequately handling edge cases or malformed inputs.
The operational impact of this vulnerability extends beyond simple service disruption, as it represents a potential vector for attackers to compromise the stability of virtualized environments. While the QEMU security team has disputed the severity of this issue by noting that privileged guest users already possess numerous methods to achieve similar denial of service effects, the vulnerability remains significant from a security engineering perspective. The flaw exposes a design weakness in how QEMU handles DMA transfer validation, particularly in scenarios where guest operating systems can manipulate storage I/O operations. This weakness could potentially be combined with other vulnerabilities or used as part of a broader exploitation strategy targeting virtualization platforms. From a compliance standpoint, this vulnerability could be categorized under CWE-248, representing an unchecked exception, and may align with ATT&CK techniques related to privilege escalation and denial of service within virtualized environments.
The disputed nature of this vulnerability by the QEMU security team reflects ongoing debates within the cybersecurity community regarding the true impact of certain flaws in virtualized systems. While the specific assertion failure may not represent a critical security breach in terms of privilege escalation or data compromise, it does demonstrate that virtualization platforms must account for all possible guest interactions that could lead to process termination. This vulnerability serves as a reminder of the complex security considerations inherent in virtualization environments where the boundary between guest and host systems must be carefully maintained. The issue highlights the importance of robust error handling and input validation in hypervisor components, particularly those handling storage I/O operations that are frequently accessed by guest operating systems. Organizations utilizing QEMU virtualization should consider this vulnerability as part of their broader security posture assessment, particularly when evaluating the potential for guest-based attacks that could impact host system stability and availability.